1. Field of the Invention
The invention relates in general to a tap tool, and in particular, to a tap tool designed for threading with low torque internal holes, whereby the tap is manufactured from a molybdenum-enriched high-speed steel including a shank with a straight flute form having a positive rake angle and a short fluted section at an angle to the tap's axis, and for use when tapping ferrous materials coated with a transition metal nitride, carbide or oxide, and coated with an optional friction-reducing top layer, and for use when tapping non-ferrous materials coated with a layer containing carbon.
2. Description of the Related Art
Mechanisms and machine components requiring screw threads have a long history in technology. Specifically, the application of screw threads as fastener components dominates all other means to join parts into assemblies. Although there are many ways to generate screw threads both internal as well as external, experience has shown that taps are the favored means to generate the internal screw thread. There currently exist two tapping methods to generate internal screw threads. The dominant tapping method is by cutting and removing material from the walls of a hole to produce a helical screw thread. Alternatively, internal screw threads can be created by displacing material to cold form an internal screw thread.
Considering the method of forming internal screw threads by displacing material, numerous designs exist of so-called forming taps. However, thread forming taps inherently require much higher torque than cutting taps, and form a poorly shaped thread profile at the minor diameter. For these reasons, forming taps are not favored in many tapping applications.
Cutting taps take on many geometrical forms; the primary difference is the configuration of the flutes. The original cutting tap form is the straight fluted tap that characteristically has straight flutes parallel to the axis of the tap. Although this design works well when the chips produced by the tap are discontinuous and short, the design does not work well in ductile steel alloys that have continuous chips that bind in the flutes of the tap. This problem is severe when tapping deep holes, and frequently results in tap breakage. An alternative tap design has a helical spiral flute that forces the chip out of the hole in a direction that depends on the direction of the helical rotation of the flute relative to the thread rotation. This design prevents chips building up in the flutes of the tap, except when friction-reducing and life-enhancing coatings, such as titanium nitride, are used. Such friction-reducing coatings produce thin, long continuous chips that are not easily ejected by the helical flutes and the chips subsequently pack the flute, frequently causing the tap to break.
Taps with a combination of straight flutes and short angular flutes at the starting chamfered end of the tap are the most effective cutting tap design for tapping through holes. Known in the art as spiral pointed taps, this design allows the chip to be forced ahead of the motion of the tap and effectively avoids chip packing problems that occur when tapping deep holes with straight fluted or spiral fluted taps. In such existing art, the straight flutes of spiral pointed taps may either have a symmetrical form with a constant radius, or a more complex asymmetrical form that consists of a straight cutting face and multiple radii. These conventional taps with a symmetrical flute form typically have a negative chordal hook, as shown in FIG. 6, which results in cutting with higher torque when the tap wears down. In addition to effectively tapping through holes, spiral pointed taps can also be used to tap blind holes with sufficient clearance for chip accumulation at the bottom of the hole.
It is well known that cutting tools generate temperatures that are high enough to limit the life of the tool, thereby reducing the effective cutting speed that can be used. The temperature that is generated during cutting or forming depends on the frictional properties between the tool and the work material. The wear rate can be reduced and the performance of cutting tools can be improved by reducing friction and consequently temperature. Additionally, the risk of softening high-speed steel tools by over tempering can be avoided by reducing cutting temperatures.
Taps may be manufactured from a variety of heat and wear resistant materials: low alloy tool steels, high alloy high speed steels, and cemented tungsten carbide. High speed steels are either tungsten enriched, whereby the major alloying element is tungsten; or molybdenum enriched, where the major alloying element is molybdenum. Tungsten enriched high-speed steels have numerous disadvantages including the limited availability of tungsten, high hardening temperatures and greater risk of decarburization during heat treatment.
Metals such as aluminum or silicon and transition metals, such as Ti, V and Cr (elements from Groups IVa, Va, VIa in the Periodic Chart) form compounds with the elements B, C, N and O. Because these boride, carbide, nitride and oxide compounds have extremely high melting points, they are refractory. They are commonly used to coat cutting tools, including taps, because of their high-temperature strength (hardness), abrasive wear resistance, extreme chemical stability and limited solubility in the work material.
There are numerous spiral pointed taps designed with an overall length that conforms to DIN standard 376 with an ASME B94.9 shank diameter. These taps are made from molybdenum-enriched high-speed steel and may have an optional coating of metal nitride, carbide or carbonitride. However, these taps do not include an asymmetrical straight flute that reduces torque when cutting is extended past the tap's chamfered section as the tap wears. Other taps that conform to the DIN standard 376 overall length and an ASME B94.9 shank diameter may be made of tungsten-enriched high-speed steel, instead of molybdenum-enriched high-speed steel. However, because these taps are made of tungsten-enriched high-speed steel, these taps have the disadvantage of the limited availability of tungsten, higher density, decreased hardenability and higher required hardening temperature during heat treatment. Therefore, these taps have limited utility.
In addition, other known spiral pointed taps have an overall length and shank diameter that both conform to ASME B94.9. These taps include an asymmetrical straight flute form having 8–12° rake angle, an angular spiral point having a 10–14° chordal hook angle, and these tap embodiments optionally included a coating of titanium nitride or titanium carbonitride. However, the prior art taps described in this paragraph do not have an overall length according to DIN standard 376, and thus cannot access on computer numerically controlled (CNC) machine tools tapped holes on parts with complicated shapes or deeply recessed holes. As a result, these taps have limited utility. It is desirable to manufacture a tap that lowers tapping torque to improve the performance of the tap, as well as to extend the useful life of the tap.